1. Field of the Invention
This invention relates to powder metal articles and their manufacture and in particular to a main bearing cap in which the body is made of a high strength powder metal material and other parts are made with a different more machinable powder metal material which is metallurgically bonded to the body.
2. Discussion of the Prior Art
FIG. 1 illustrates a basic shape of a conventional main bearing cap (MBC). It is essentially a solid body with a semi-cylindrical recess and bolt holes for mounting. MBCs are used to retain the shell bearings and crankshaft of internal combustion engines. This is accomplished by bolting the MBC A (FIG. 2) to the lower end of the engine block B (FIG. 2). The semi-cylindrical recess C in the MBC corresponds to a similar semicylindrical recess D in the engine block so as to form a round hole when bolted together as shown in FIG. 3.
In multiple cylinder engines, there are multiple MBC/cylinder block holes which are bored-out to a precise diameter E (FIG. 4) to accept bearing shells F (FIG. 5), which in turn locate and retain the crankshaft G (FIG. 5) in place. This boring operation is critical since the roundness and diametral precision have a significant effect on engine noise. A slightly oversize diameter allows the crankshaft to vibrate during operation. This is heard as a xe2x80x9crumblexe2x80x9d that is unacceptable in modern passenger vehicles. A slightly undersize or out of round bore can cause binding of the crankshaft, preventing smooth low friction operation.
Traditionally, the cylinder block has been made from either grey cast iron or an aluminum alloy, and the MBCs made from either grey cast iron or ductile cast iron (also called spheroidal or nodular cast iron). In recent years, a new material process combination has become commercially applied, namely a sintered powder metal (P/,M) steel. A principal cost advantage of the P/M steel is the near net shape that can be achieved, which minimizes machining and associated costs of the product.
However, since the MBC has to be bored at the same time as the cylinder block, there is a challenge related to the difference in machinability of the P/M steel and the block material (cast iron or aluminum alloy). This has lead to improvements in the P/M steel material machinability by well known and published means that include additions of machinability aids to the P/M material. This has been beneficial, but not universally successful in matching the MBC and block material machinability. A further issue is the ever increasing development of engine technology that continues to try to obtain more power from smaller (lower weight) and faster turning engines to extend fuel economy and performance respectively.
A natural extension of the MBC technology to handle this added power-density and higher loads is to raise the strength of the P/M steel. This requires that the P/M steel be strengthened by some means, such as heat treatment, as is practiced in conventional steel technology. Heat treatment involves production of a stronger but also much harder steel which is difficult to bore, and results in very short lived and expensive cutting tooling. The short tool life means interrupted engine production on very costly automated machining lines.
Thus, a need exists to match the machinability of the recesses C and D while maintaining a high strength, low weight bearing cap of near net pre-machined shape and dimension.
The invention provides a two material bearing cap made by powder metallurgy. In one aspect, there is a thin layer of a more machinable material lining the half circular bore of the bearing cap, with a stronger, harder material forming the majority and rest of the body of the bearing cap.
In this aspect, a bearing alloy composition may be used for the softer material in the bore arch region, which may be line bored to produce an integral bearing surface, thereby eliminating the shell bearings which otherwise are used. The integral bearing bore is only possible in bearing caps used in aluminum engine blocks. Whether the more machinable powder material lining the half bore of the bearing cap is a bearing alloy or not, line boring is facilitated and tool life is prolonged by the invention.
Also in this aspect, the material lining the half circular bore of the bearing cap is preferably chosen to match the machinability of the half circular bore of the engine block.
In another aspect, there may be provided a two material bearing cap made by powder metallurgy where there is a thin layer of a softer material on each joint face of the foot of the bearing cap, including where present, integral dowels, with a stronger harder material forming the majority and rest of the body of the bearing cap. By forming the dowels of a softer material, they are more conformable to the counterbore in the engine block in which they are pressed, and better repressed into them during the fit-up and installation of the crankshaft.
In another aspect, the bearing cap may be made with outboard wings for cross-bolting. These wings may be made from a softer, more machinable material, with a stronger harder material forming the majority and rest of the body of the bearing cap.
Each of these aspects may be used alone, or in any combination with one or more of the other aspects.